1. Field of the Invention
This invention relates generally to a unique method for manufacturing a dry-formed, adhesively bonded, nonwoven fibrous sheet and to the sheet formed thereby. Specifically, this invention relates to a unique method for manufacturing a dry formed, adhesively bonded and creped nonwoven fibrous sheet having a high degree of bulk, flexibility, extensibility and porosity so as to permit the sheet of this invention to be used as a substitute for other nonwoven materials and conventional textile materials in absorbent products.
2. Description of the Prior Art
Dry-formed, nonwoven fibrous sheets have become exceedingly popular, especially for single or limited use applications, to replace higher cost textile webs formed by conventional textile operations, such as by weaving and knitting. Obviously, it is highly desirable to form the nonwoven sheets in a manner which will impart properties thereto that are similar to those possessed by the conventional textile fabrics which they are intended to replace. Specifically, it is desired to form such nonwoven sheets in a manner to impart sufficient bulk, flexibility and extensibility thereto for approximating the "hand" of such textile fabrics. Also, for many applications the nonwoven sheets should be absorbent and/or porous. Absorbent and/or porous nonwoven sheets are commonly employed as replacements for conventional textile fabrics as household and industrial wipers, as well as cover sheets for sanitary napkins and disposable diapers.
The initial step in the formation of dry-formed, nonwoven fibrous sheets is to form a dry fibrous web having a low integrity. It is often difficult to sustain the integrity of this web as it is directed through subsequent processing operations, such as, gravure printing operations and various post-treating operations, i.e., embossing, compacting, winding and the like.
To overcome the processing problems associated with the handling of low integrity, dry formed webs, it has been suggested in the prior art to initially strengthen the web by the inclusion of pre-bonding material, which may be either of a temporary type, or of a permanent type, for permitting the subsequent post-treating operations to be carried out without web failure. In these prior art methods, either the type and/or amount of the pre-bonding material is chosen to avoid any negative effect on the desired properties in the completed sheet (e.g., stretch, flexibility, absorbency, etc.), or, if the type of bonding material does adversely affect the properties of the completed sheet, it is washed out.
U.S. Pat. No. 3,676,245, issued to Helmut et al., discloses the temporary strengthening of a continuous filament web, prior to the final bonding thereof, by directing fine droplets of water onto the web surface. The surface tension of the water temporarily strengthens the freshly formed web so that it can be directed through the subsequent processing operations of adding a binder to the web and heating and drying the web for hardening or setting of the binder. Upon drying of the web the water which was initially applied to temporarily strengthen it will have been dissipated, and therefore will not provide any negative effect on the properties associated with the completed sheet.
In U.S. Pat. No. 2,719,802, issued to Nottebohm, a prebinder, in the form of a resin foam, is applied to a web surface to provide light surface cohesion prior to adding additional binder to the web to complete its structure. The bonding materials suggested for use in the Nottebohm patent are not of the type which form brittle bonds, and therefore may not adversely affect the flexibility, softness and drape of the completed sheet, unless an excess quantity of adhesive is applied.
In U.S. Pat. No. 3,485,695, issued to Ness, both the pre-bonding material and the final bonding material are elastomeric, and both create permanent bonds in the completed web. These binder materials are chosen so as not to adversely affect the formation of a web having textile-like properties, i.e., flexibility, softness and drape.
It is known to employ temporary binders of the type contemplated for use in this invention, to temporarily bond a dry-formed web so that said web can be further processed, as evidenced by the disclosures in U.S. Pat. Nos. 2,865,783, issued to Henderson et al. and 2,545,952, issued to Goldman.
Henderson et al. disclose washing out the brittle, temporary bonds between fibers by passing the web through a hot water bath after said web has been completely formed. The addition of water to the completed web is undesirable since additional drying equipment is required to remove the water, and this adds to the cost and complexity of the manufacturing process.
Goldman discloses a spot bonded, dry-formed web, and indicates that for some applications a small amount of starch can be added to facilitate the processing of the web. Goldman indicates that the inclusion of small amounts of starch is not inconsistent with the formation of a flexible web so long as the web is capable of flexing. However, a dry-formed sheet including brittle starch bonds therein does provide some degree of stiffness, as well as an undesirable hand in the completed sheet.
Print bonded fibrous webs of general interest to the subject matter of the instant invention are disclosed in U.S. Pat. Nos. 2,705,498, issued to Johnson; 2,958,608, issued to Barnard; 3,059,313, issued to Harmon; 3,236,718, issued to Cohen et al.; 3,665,921, issued to Stumpf; 3,665,922, issued to Skora and British Pat. No. 1,294,794, assigned to Scott Paper Company. In all of these patents, with the exception of the Johnson patent, a bonded fibrous web is mechanically worked as part of the formation process. However, none of these patents are concerned with, nor suggest a solution to the problem of low web integrity associated with a dry-forming operation, such as an air-laying, carding, garnetting or similar operation.